1. Field of the Invention
The present invention relates to a line source unit and an image input apparatus for use in an image input unit of a facsimile machine, a scanner, an image sensor and the like.
2. Description of the Prior Art
One example of a line source unit as a light source used in an image sensor for reading characters and images on a document or an original, is described in the Japanese Laid-Open Patent Publication No. 8-163320. FIGS. 13 to 16 schematically show a bar-type lighting apparatus and a document reading apparatus (i.e., an image sensor) using the bar-type lighting apparatus, disclosed in the above Japanese Laid-Open Patent Publication.
FIG. 13 is an exploded perspective view of a conventional line source unit. FIG. 14 shows the line source unit from which light of line-shaped being emitted. FIGS. 15A and 15B show a light generation device used in the line source unit, where FIG. 15A is a front view of the device and FIG. 15B, a side view. FIG. 16 is a sectional view of an image sensor (a contact-type image sensor) using this line source unit.
The line source unit and the image sensor shown in FIGS. 13 to 16 include a light generation device 1 for generating a line-shaped light, a light guiding member 2, a cover 3, a resin molded member 4 partly forming the light generation device 1, pins 5 and a sensor board 6. The light guiding member 2 conducts light from the light generation device 1 onto an original. Part of the light passing through the light guiding member 2 but not directing toward the original, is reflected again by the cover 3 so as to direct the light toward inside the light guiding member. The cover has a tone with a large reflection rate such as that of white. The pins 5 electrically connect the light generation device 1 and the sensor board 6 of the image sensor.
The line source unit and the image sensor further include a plurality of LED chips (bare chips) 7 mounted on the resin molded member 4, a sheet copper 8 which electrically connects the LED chips 7 and the pins 5 and has these LED chips 7 on it, a glass plate 10 arranged to the position where an original 9 travels, and a rod lens array 11 for an erected-equimagnification-image formation consisting of a plurality of rod lenses (not shown).
It should be noted that sensor ICs 12 are rectilinearlly placed on the sensor board 6. Frame 13 positions each of the above-described elements, and the LED chips 7 have a sealing material 14 for protecting themselves from oxidation and an external force.
The operation of the line source unit and the image sensor is now explained. Anode and cathode (not shown) of the LED chip 7 are respectively connected to the pins 5 via the sheet copper 8. These pins 5 are further connected to the sensor board 6, and the sensor board 6 itself is electrically connected to the external part of the image sensor. When an external voltage is applied to the anode and cathode of the LED chip 7 through the sensor board 6 and the pins 5, the LED chips 7 generate light. The light then enters inside the light guiding member 2 which is equipped to neighbor these chips.
The light is reflected by a light diffusing layer formed on the light guiding member 2. The light diffusing layer is formed by printing a color of white on the guiding member. In another application, the layer has the small convex and concave (not shown) which are built up on the light guiding member 2 itself. The light guiding member 2 therefore emits uniform light in the direction as indicated with arrows in FIG. 14.
In the image sensor shown in FIG. 16, light coming from the light guiding member 2 illuminates the original 9 through the glass plate 10. The illuminated light is reflected by the original in accordance with gradation of images contained in the original, and the reflected light travels through rod lenses located on the rod lens array 11, thus forming an image on the sensor ICs 12. Though each sensor IC 12 is several millimeters in length, a plurality of the sensor ICs 12 are arranged in line so that they achieve conformity with a readout width of the image sensor. These sensor ICs 12 store electric charge in accordance with the strength of an incoming reflected light, and output the electric charge through the sensor board 6. The frame 13 physically supports the rode lens array 11, the glass plate 10, and the sensor board 6 which constitute the image sensor.
The prior art line source unit has a problem that if the unit is determined to be defective after it is manufactured, because of the fact that only one of the LED chips 7, for example, does not light properly, the chip can neither be repaired nor replaced with a good one. Therefore, the defective chip can not help being discarded together with the remaining non-defective LED chips. Furthermore, similar to the above case, if one or more of the LED chips 7 after being shipped as a final product or installed in the image sensor, do not light due to deterioration or its operational life, all of the chips installed have no choice but to be discarded.
Especially, since the LED itself is a bare chip with a size of several tens of micrometer (.mu.m) square, there is a disadvantage that it is difficult to determine whether the chip is defective or not before it is implemented on the sheet copper 8. This requires to check whether the chip lights properly or not, after the implementation on the sheet copper, thus increasing the frequency of discarding defective chips.
Taking this problem into consideration, a trial has been made, as shown in FIG. 17, to obtain a light generation device with LED chips 15 which are implemented on a base board 16. These chips are packaged within a transparent resin so as to use the device as a light generation device of FIG. 15A. However, this device also has the following problem (which will be described with reference to FIG. 18 and FIG. 19), and the device of this kind has never been realized.
FIG. 18 shows the amount of light obtained by the light guiding member 2 in which the LED chips 15 are used. The abscissa is a distance from the LED (the nearer the left side of the abscissa, the closer to the LED). The ordinate is the amount of light obtained. FIG. 19 shows how light emitted by the LED chips 15 travels in a line source unit, i.e., an optical path in the unit, and FIG. 19 also describes a light guiding member 2, a cover 3, and a base board 16 which form the line source unit.
As shown in FIG. 19, part of the light emitted by the LED chip 15 which travels in a direction toward the wall of the light guiding member 2 at an angle smaller than the angle .theta., undergoes total reflection at the point where the light collides with the wall. The light then travels inside the light guiding member 2. .theta. is the angle at which light totally reflects and is determined by the material forming the light guiding member 2. However, part of the light propagating toward the wall of the light guiding member 2 at an angle larger than .theta. gets through the light guiding member 2 to the cover 3. The light then diffuses to illuminate an original. Therefore, at the place near the LED chip 15, there exists light which is directed toward the wall of the light guiding member 2 at an angle larger than .theta.. As a result, the amount of light tends to increase at the place near the LED chip 15, as shown in FIG. 18.
On the other hand, in the conventional line source unit described above, the LED chip 7 is small enough to be surrounded by walls of the resin molded member 4 as shown in FIG. 20, only light with a smaller incidence angle propagates inside the light guiding member 2. This causes no problem as far as the amount of light is concerned. One way to avoid such a problem as to the amount of light is to limit an area from the light source and to use light obtained from the area which is far away from the LED of the light source. However, this is against the greatest benefit of the contact-type image sensor, that is, the sensor becomes small in size and light in weight. Therefore, it is not beneficial to adopt a method of limiting the area from the light source.